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I think the 50mm lens is such a gold standard of optical design that it would be the obvious choice for distortion and field flatness, which would always be better than the equivalent 40mm.
As for f ratio, I assumed that the Nikon f1.4 would be better than the f1.8, but you have to stop both of them down to get sharp stars to the edge, so I'm not sure if the faster lens will be necessarily better for astrophotography. There are a few sites that publish performance figures for different lenses, so it might be worth looking at chromatic aberration and edge sharpness measurements if you can find it. For example - I have an older Nikon 180mm and was a little surprised how much ca it had at f5, then discovered it worked much better at f2.8. Each lens design is subtly different! Normally a lens works best around f7-f11.
Cheers, and hope it's food for thought.
Andrew.
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